1. Field of the Invention
The invention relates to detectors of the level of supply voltage Vcc of an integrated circuit.
These detectors are incorporated into the integrated circuit and can be useful for various applications such as:                detecting that the supply voltage is in a specified range for which the circuit is designed and prohibiting operation if the voltage is in another range;        detecting the range in which the supply voltage is located if there are several possible ranges, and changing the configuration of operation of the integrated circuit as a function of the detected range;        ascertaining that the supply voltage has reached a specified threshold before permitting the operation of the integrated circuit.        
Thus, for example, it may be sought to make a voltage level detector that finds out whether or not the voltage level is greater than about 2 volts and another detector that finds out whether or not it is greater than about 4 volts. These two detectors may be used simultaneously in one and the same integrated circuit which could work for several ranges of supply voltages that are different, such as for example a range of 2.7 to 3.3 volts and a range of 4.5 to 5.5 volts. The combination of information elements given by the two detectors indicates the range in which the supply voltage is located.
2. Description of the Prior Art
Such a detector, shown in FIG. 1, consists of a comparator having as input two reference voltages, Vref1 and Vref2, that vary differently as a function of the supply voltage, VCC, and that vary in such a way that their curves of variation intersect when the supply voltage reaches a specified threshold (FIG. 2). The comparator compares these two references and switches over in one direction or another depending on whether the supply voltage Vcc crosses this threshold or not. The output of the comparator may be applied through a buffer amplifier to the rest of the integrated circuit in order to modify, permit or prohibit its operation depending on the desired application.
There are two main difficulties, which affect the detection of low supply voltage levels (the detection of a threshold of 2.5 volts for example). First, it is difficult to make a comparator that works accurately even when it is supplied with a low value of supply voltage (far below the threshold to be detected). Second, it is difficult to make reference voltages that meet the above conditions (regarding different variations as a function of Vcc, and threshold value of intersection of their variation curves) because each reference voltage varies as a function of Vcc which depends firstly on the operating temperature of the integrated circuit and secondly on the variations of parameters of the method of manufacture of this circuit.
Consequently, whereas it is sought to have reference voltages whose curves of variation intersect at a well-defined point that corresponds to a desired threshold value Vs, it is observed in reality that it is necessary to plot a quadruple network of curves that intersect in a zone of threshold values which may be very extensive. This quadruple network consists of two networks of curves for the first reference voltage Vref1 and two networks for the second reference Vref2. For each reference, a network of curves may be plotted as a function of the possible variations of the manufacturing method and another may be plotted as a function of the operating temperature of the circuit.
It can be easily understood that, with this quadruple network, the variation of the threshold voltage as a function of manufacture and as a function of the temperature becomes great and makes the detector of little use and of little reliability.
This is all the truer as the curves of variation of the reference voltages intersect with a narrower acute angle. For, the greater the manufacturing and temperature variations, the more variable will be the position of the intersection.
FIG. 3 gives an exemplary illustration of the different points of intersection of a curve Vref2 with several curves Vref1 corresponding to a certain degree of variation of manufacturing parameters and/or a variation of operating temperature. The result thereof is uncertainty in regard to the significance of the output information from the comparator for it corresponds to a crossing of a threshold Vs which may vary within a fairly broad range.
There is therefore need for a detector whose detection threshold is as stable as possible despite variations in manufacture and despite variations in operating temperature. This detector must be simple and must consume little current.